[flang][docs] Document the `flang` wrapper script

[kiranchandramohan]

LGTM.

[Meinersbur]

LGTM

flang/docs/FlangDriver.md

150

flang/docs/FlangDriver.md

 
 The frontend driver glues all of the frontend libraries together and provides
 an easy-to-use and intuitive interface to the frontend. It accepts many
 frontend-specific options not available in `flang-new` and as such it provides a
 finer control over the frontend. Similarly to `-Xclang` in `clang`, you can use
 `-Xflang` to forward the frontend specific flags from the compiler directly to
 the frontend driver.
 
+
 ## Compiler Driver
 
 The main entry point for Flang's compiler driver is implemented in
 `flang/tools/flang-driver/driver.cpp`.  Flang's compiler driver is implemented
 in terms of Clang's driver library, `clangDriver`. This approach allows us to:
 * benefit from Clang's support for various targets, platforms and operating systems
 * leverage Clang's ability to drive various backends available in LLVM, as well
   as linkers and assemblers.

 invoking the compiler driver, `flang-new`, with the `-fc1` flag.
 
 The frontend driver will only run one action at a time. If you specify multiple
 action flags, only the last one will be taken into account. The default action
 is `ParseSyntaxOnlyAction`, which corresponds to `-fsyntax-only`. In other
 words, `flang-new -fc1 <input-file>` is equivalent to `flang-new -fc1 -fsyntax-only
 <input-file>`.
 
+## The `flang` script
+The `flang` wrapper script for `flang-new` was introduced as a development tool
+and to facilitate testing. While code-generation is not available in Flang, you
+can use it as a drop-in replacement for other Fortran compilers in your build
+scripts.
+
+The `flang` wrapper script will:
+* use `flang-new` to unparse the input source file (i.e. it will run `flang-new
+  -fc1 -fdebug-unparse <input-file>`), and then
+* call a host Fortran compiler, e.g. `gfortran`, to compile the unparsed file.
+
+Here's a basic breakdown of what happens inside `flang` when you run `flang
+file.f90`:
+```bash
+flang-new -fc1 -fdebug-unparse file.f90 -o file-unparsed.f90
+gfortran file-unparsed.f90
+```
+This is a simplified version for ilustration purpose only. In practice, `flang`
+adds a few more frontend options and it also supports various other use cases
+(e.g. compiling C files, linking existing object files). `gfortran` is the
+default host compiler used by `flang`. You can change it by setting the
+`FLANG_FC` environment variable.
+
+Our intention is to replace `flang` with `flang-new`. Please consider `flang`
+as a temporary substitute for Flang's compiler driver while the actual driver
+is in development.
+
 ## Adding new Compiler Options
 Adding a new compiler option in Flang consists of two steps:
 * define the new option in a dedicated TableGen file,
 * parse and implement the option in the relevant drivers that support it.
 
 ### Option Definition
 All of Flang's compiler and frontend driver options are defined in
 `clang/include/clang/Driver/Options.td` in Clang. When adding a new option to